Data storage devices, such as disk drives and solid state drives are employed in numerous areas such as computer systems (e.g., desktops, laptops, portables, etc.) and consumer devices (e.g., music players, cell phones, cameras, etc.). User data is typically stored in a non-volatile memory, such as a magnetic disk or a non-volatile semiconductor memory (e.g., Flash memory). It is desirable to detect as soon as possible when a power failure occurs so that protective action can be taken, such as flushing cached data to the non-volatile memory and/or parking a head on a ramp.